Principle And Method Of Global Generation Of Cutter Location Paths For Complex Surface Machining Driven By The Streamline Field

Complex curved parts such as blisks and blades are key parts of high-end equipments like aero engines and gas turbines.They are mostly used in unconventional working conditions of high temperature,high pressure and strong impact,and the machining accuracy and surface quality of which have become one of the most important constraints in improving performance of high-end equipment.Five-axis CNC machining is the mainstream technology for machining of complex curved parts such as blisks and blades,however,how to improve the level of CNC programming technology under the existing equipment conditions to achieve improvement of both surface quality and processing efficiency,has always been the core difficulty in machining parts like these.With the continuous improvement of part design requirements and service performance,parts not only generally have a compact structure,but also often show complex features such as bending,sweeping,and torsion,resulting in poor tool accessibility,abrupt tool axis changes and integrity of the machined surface hard to predict and control,then it challenges the theory and method of precision machining of complex curved parts.However,the tool paths planned by the general software package mostly focuses on the geometrical level,simply taking the shortest path length or the largest path interval as the machining goal,lacking consideration of tool path kinematics and machine tool driving characteristics.Moreover,the design of a single path and the transition of paths in adjacent areas often adopt a sequential way,short of an overall path design and transition method,and therefore,it is difficult to realize the high-performance five-axis tool path planning requirements to ensure machining accuracy,efficiency and surface quality.In order to solve these problems,this paper has carried out the research on the principle and method of global generation of cutter location paths for complex surface machining driven by the streamline field.A unified framework for planning tool paths suitable for different surface types is designed,and a streamline tool path generation method that integrates both geometric and physical characteristics in machining is proposed,so as to to provide theoretical basis and technical support for solving the difficulties in machining complex surface parts represented by blisks and blades with high efficiency and high quality.The main contents of this paper are organized as follows:(1)The overall generation strategy and method of streamline tool paths in single pararneter domain are studied.We first determine the 3D feed directions at the sampling points based on the processing intention,and then use the differential geometry to obtain the dimensionalityreduction mapping result of these 3D directions and construct a discrete feed direction vector field in the 2D parameter space.On this basis,according to the plane flow field theory,a continuous direction field model based on B-spline is established,and the differential description of a streamline trajectory is given;Moreover,based on the direction information inherent in the vector field,an efficient discrete strategy of a streamline tool path under given accuracy is proposed.The machining strip width of a streamline trajectory in the parameter domain is also revealed,and the arrangement rule of streamline paths under the constraint of scallop height is established.Finally,the overall planning of streamline tool paths in a single parameter domain is realized.(2)A unified streamline-field modeling method in multiple parameter domains is proposed.For the adjacent streamline fields,based on the component expression of a streamline path tangent along different parameter directions at the parameter domain boundary,the G1 continuity condition for smooth transition of streamline paths is given.An optimization model of the stream function control point sequence with the goal of minimizing streamline changes is established,and the Lagrangian algorithm is used for the optimal result.On this basis,for the smooth blending of multiple adjacent streamline fields,a linear equation set for adjusting the control point sequence is constructed to meet the compliant requirements,avoiding the problem of corner incompatibility,which may occur in the G1-continuity blending process of multi-streamline fields with a common corner.Further,the generation method and arrangement criterion for streamline tool paths in multiple parameter domains under a unified vector field model is provided.The proposed method directly completes the generation and blending of tool paths from a global perspective,and solves the problem that the geometric behavior of tool paths difficult to control as a whole when they are planned in a traditional one-by-one path generation manner.(3)An efficient tool positioning model without gouging is established for a streamline tool path.An algorithm of adaptively sampling discrete points on a tool axis,and the collision conditions of the cutter shaft are both given.For collision cutter locations,starting from the inherent correlation of cutter contact point,cutter tip point and tool axis,an efficient adjustment method for the critical collision-free tool axis under the condition that the cutter contact point remains unchanged is proposed,and thus the construction of accurate collision-free feasible region is realized.On this basis,according to the osculation machining theory,the tool osculation curve is determined,and the criterion of the initial tool orientation that maintains high-order contact in the collision-free region is proposed.The overcut detection algorithm based on the shortest distance between the cutting part of a cutter and the design surface is given,and then,a tool axis adjustment model for overcut elimination is constructed to achieve two-point contact between the tool and the surface without overcuts.The proposed tool location method provides an effective way for efficiently planning five-axis streamline tool paths.(4)A local tool axis optimization method for five-axis streamlined tool path under multisource constraints is proposed.Taking the five-axis streamline tool path after initial tool positioning as input,the rotation angle sequence of a machine tool is obtained through kinematic transformation,and the adjustment range in the tool path is determined with the help of kinematic analysis.Therefore,starting from the geometric and kinematic requirements in the adjustment of tool axes,using the strategy of divide-and-conquer optimization,the gouge-free adjustment boundaries and the linear expression of the rotary angular velocity and angular acceleration of a machine tool are obtained respectively,based on the feasible region search and numerical difference techniques;The equivalent conversion equation of the minimum square sum of rotary axis acceleration under the specified boundary tool axes are proposed.Furthermore,a quadric optimization model with motion smoothing as the goal,no gouging and complaint with the kinematic requirements is established.On this basis,the formation mechanism of machine tool singular motions is revealed and the machine tool singularity avoidance is realized by relying on the proposed local tool axis optimization model.The machining example shows that tool paths optimized with the proposed method can avoid the singularity of a machine tool and realize the smooth motion without gouging.(5)The processing verification experiment of titanium-alloy blisk was implemented.Taking the blisk model roughed by abrasive water jet as the milling object,the technical process of blade modification,blade machining,blade root cleaning and hub finishing is given.Based on the proposed streamline tool path planning method,the streamline tool path with geometric smoothness and satisfying the kinematics of a machine tool is generated.Aiming at meeting the requirements of machining rigidity,the machining plan composed of blade partition and subsequent milling is proposed,and the performing sequence of tool paths that remove material in each blade section is scheduled,developing a semi-finishing and finishing mixed cutting process.Finally,the surface roughness of the machined blisk was tested,which verified the effectiveness of the proposed process route.